This invention relates to a novel method of making a dipolar-deflecting and quadrupolar-focusing color-selection structure for a CRT (cathode-ray tube).
A commercial shadow-mask-type color television picture tube, which is a CRT, comprises generally an evacuated envelope having therein a target comprising an array of phosphor elements of three different emission colors arranged in color groups in cyclic order, means for producing three convergent electron beams directed towards the target, and a color-selection structure including a masking plate between the target and the beam-producing means. The masking plate shadows the target, and the differences in convergence angles permit the transmitted portions of each beam, or beamlets, to select and excite phosphor elements of the desired emission colors. At about the center of the color-selection structure, the masking plate of a commercial CRT intercepts all but about 18% of the beam current; that is, the plate is said to have a transmission of about 18%. Thus, the area of the apertures of the plate is about 18% of the area of the masking plate. Since there are no focusing fields present, a corresponding portion of the target is excited by the beamlets of each electron beam.
Several methods have been suggested for increasing the transmission of the masking plate; that is, increasing the area of the apertures with respect to the area of a plate, without substantially increasing the excited portions of the target area. In one approach, the apertures are arranged in columns opposite substantially-parallel phosphor stripes in the target. Each aperture in the masking plate is enlarged and split into two adjacent windows by a conductor. The two beamlets passing through adjacent windows are deflected towards one another, and both beamlets fall on substantially the same area of the target. In this approach, the transmitted portions of the beams are also focused in one transverse direction and defocused in the orthogonal transverse direction.
One family of CRTs employing such a combined deflecting-and-focusing color-selection structure includes, as normally viewed, a target comprising a mosaic of vertical phosphor stripes of three different emission colors arranged cyclically in triads (groups of three different stripes), means for producing three convergent horizontally in-line electron beams directed towards the target, and a color-selection structure located adjacent and closely spaced from the target. The color-selection structure comprises a metal-masking plate having therein an array of substantially-rectangular apertures arranged in vertical rows and an array of narrow vertical conductors insulatingly spaced and supported from one major surface of the masking plate, with each conductor substantially centered over the apertures of one of the columns of apertures. Each conductor is unsupported and uninsulated over each aperture. Viewed from the electron-beam-producing means, the conductors divide each aperture into two essentially-equal horizontally-coadjacent windows.
When operating this latter device, the narrow vertical conductors are electrically biased with respect to the masking plate, so that the beamlets passing through each of the windows of the same aperture are deflected horizontally toward the positively-biased side of the window. Simultaneously, because of the quadrupole-like focusing fields established in the windows, the beamlets are focused (compressed) in one direction of the phosphor stripes and defocused (stretched) in the other direction. The spacings and voltages are so chosen to form an array of electrostatic lenses that also deflect adjacent pairs of beamlets to fall on the same phosphor stripe of the target. The convergence angle of the beam that produces the beamlets determines which stripe of the triad is selected.
In a typical color-selection structure of this type, the apertures may be about 0.625 mm (25 mils) wide on about 0.760 mm (30 mils) centers horizontally and about 0.300 mm (12 mils) high on about 0.450 mm (18 mils) centers vertically, and the conductors may be about 0.075 mm (3 mils) wide and spaced about 0.050 mm (2 mils) from the plate. Because of the small and precise sizes required of the apertures and of the strips, special techniques must be employed to fabricate structures of this type at reasonable cost. Several methods have been suggested previously. But, each prior method appears to be too costly and may not produce an adequate yield of acceptable structures.